Consumer stereo audio systems implementing surround sound typically extract from two stereo channels (Right and Left) an additional channel which contains the out of phase or ambience information which is contained in the original two channels. This ambience information is obtained by deriving the difference in the signal content between the right and left channels in a stereo system. This ambience information can either be fed through a single rear channel to a single speaker or, to spread the information across the full rear of the sound field, be fed through two rear channels with one channel being 180 degrees out of phase with the other to two rear speakers.
The signal processing required to do this is simply a subtraction of the left channel from the right channel. The compliment is done to extract the second rear channel, that is, the right channel minus the left channel.